How to Use the Grounding Wire Size Calculator
The grounding electrode conductor (GEC) is the wire that safely carries fault current to earth if a ground fault occurs in your electrical system. If it's undersized, it can overheat or fail during a fault instead of doing its job, so the National Electrical Code (NEC) sizes it based on the largest ungrounded (service-entrance) conductor feeding the system. This tool lets you select your service conductor size and instantly see the minimum copper grounding electrode conductor size required under NEC Table 250.66.
Table 250.66 groups service conductor sizes into bands: for example, a 2/0 or 3/0 AWG service conductor requires at least a 4 AWG copper grounding electrode conductor, while anything over 1,100 kcmil requires at least 3/0 AWG. As service conductors get larger, the required grounding conductor grows too, but not at a strict 1:1 ratio — the table reflects decades of testing on realistic fault current levels.
These are minimum sizes, so using a larger grounding conductor than required is always acceptable and sometimes preferred for mechanical durability. What's not acceptable is undersizing it — an undersized grounding electrode conductor is a common finding in electrical inspections and should always be corrected to at least the Table 250.66 minimum by a licensed electrician.
Frequently Asked Questions
During a ground fault, a large fault current can flow through the grounding electrode conductor briefly. It needs to be sized proportionally to the service conductor so it can safely carry that fault current to earth. NEC Table 250.66 sets these minimum sizes.
Yes. Table 250.66 lists minimum sizes, so using a larger conductor is always acceptable from a safety standpoint — installers usually just balance that against cost and ease of installation.